This invention relates to semiconductor devices and, more particularly, to semiconductor devices, including wide bandgap semiconductor devices, having improved contacts and methods of fabricating the same.
Semiconductor materials such as silicon (Si), gallium nitride (GaN), and gallium arsenide (GaAs) have found wide application in semiconductor devices for consumer, commercial and other applications. A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT). A HEMT is a field effect transistor (FET) that incorporates a junction between two materials with different bandgaps (i.e., a heterojunction) as the channel, instead of a doped region, as is generally the case in integrated circuit field effect transistors. A Schottky barrier gate is used to control a two-dimensional electron gas (2DEG) between a source region and a drain region. A Schottky barrier is a potential barrier formed at a metal-semiconductor junction, which has rectifying characteristics.
Wide bandgap HEMT and FET devices generally include a wide bandgap semiconductor layer and a gate electrode on a planar surface of the wide bandgap semiconductor layer. A gate contact may be provided that is electrically connected to the gate electrode that is used to supply a voltage to the gate electrode. The gate contact may be formed outside the active area of the device, and may be formed integrally with the gate electrode. Both the gate electrode and the gate contact typically each include three metal layers: a barrier metal layer in contact with the wide bandgap semiconductor, a current spreading layer, and a diffusion barrier layer therebetween to reduce or prevent diffusion of the current spreading layer into the metal layer. For example, nickel is the most commonly used barrier metal layer for GaN-based HEMTs. The current spreading layer is typically gold and the diffusion barrier layer is typically platinum. The gold current spreading layer serves to enhance the conductivity of the gate electrode and to lower gate electrode resistance. The platinum diffusion barrier layer is used as a barrier for gold diffusion, which, if allowed to reach the semiconductor surface, may create a degraded Schottky contact.
One step in the fabrication of such wide bandgap devices is the formation of the gate electrode. Conventional methods of gate electrode formation may include depositing a dielectric on the wide band gap semiconductor, etching through the dielectric using a mask and/or other sacrificial layer to expose a planar surface of the underlying wide band gap semiconductor, and depositing the metal layers in a T-shape (when viewed from the top) onto the exposed planar surface of the wide band gap semiconductor thereby forming the gate electrode and the gate contact (referred to as a “T-gate”). Other conventional methods of gate electrode formation include depositing a dielectric on the wide bandgap semiconductor, etching through the dielectric using a mask and/or other sacrificial layer to expose a planar surface of the underlying wide bandgap semiconductor, and removing the mask and/or other sacrificial layer from the dielectric prior to formation of the gate electrode. As such, the gate electrode may completely fill the etched portion of the dielectric, and “wings” or sidelobes of the gate electrode may be formed directly on the dielectric surface. Such a gate electrode is often referred to as a “gamma” gate.